The basis for the evaluation of impact significance must be clearly set out for each topic in worksheets or supporting documentation. In most cases, impact significance is a function of the two variables, impact magnitude and receptor sensitivity. For example, a small-scale option in an area of unremarkable landscape may not be significant in terms of landscape quality whereas the same option in a National Scenic Area may be evaluated as having a major impact. Small increases in noise levels may not be significant where noise levels are already high, but could be significant in a quiet rural village. The AST allows for specific sensitivities to be recorded in the ‘qualitative' column and more information about these should be provided in worksheets.
In line with the approach of the DfT, it is proposed that the seven point scale be adopted for assessing the predicted magnitude of impacts on each of the sub-criteria - the appropriate level should be recorded in a worksheet or in supporting documentation:
At the strategic level, this assessment may need to be based entirely on qualitative information, with informed value judgements recorded in the ‘qualitative' column of the AST. In some cases, the consideration of options will be informed by the results of traffic models. Such models would allow a quantitative approach to be adopted to the assessment of effects on noise and air quality. Where this information is available, it should be recorded in the ‘quantitative' column of the AST. At the project level, both ‘qualitative' and ‘quantitative' columns should be completed. Those entries should provide a summary of the data included in worksheets and other supporting documentation.
Once the magnitudes of impacts have been identified, impacts must be evaluated so that their significance can be determined. It is proposed that significance should be recorded in the column carrying that heading in the AST, using a seven point scale as described in Section 5.4.
The significance of impacts may be evaluated by means of expert judgement by individuals or panels, or may relate specifically to published standards and thresholds, for example in the case of air quality.
